1. Field of the Invention
The present invention pertains generally to the field of measurement of water flowing in partially and completely full pipes using a sensor that is in contact with the flow, more particularly, to the class of devices that utilize ultrasonic energy and measurement of the Doppler Effect as a means to determine the channel velocity.
2. Background of the Invention
There are many cases where it is important to measure the rate of flow in a pipe. For example, one may need to know the flow rate to determine a billing rate between two communities, to assess the rate at which rain or ground water is entering the sewage system, to design a system expansion, or to control the rate at which a holding tank is filled or emptied. In measuring such flows there are periods when the conduit may be empty of fluid, partially full or completely full. The flow may be free flowing (propelled only by the force of gravity). It may be constrained by an obstruction or other down stream constraint. It may be flowing downstream due to an upstream pressure head, or it may be flowing upstream (in a reverse direction) owing to a downstream pressure head. Flow in all of these cases is determined by the Continuity Equation:Q=AVwhere, Q is the flow rate, A is the cross-sectional area of the flow and V is the average velocity of the flow. The cross-sectional area is relatively easy to measure. One need only know the geometry of the pipe and the depth of flow. One can measure the diameter of the pipe or, if the pipe is non-circular, determine the geometry by direct measurement. There are a wide variety of depth measurement technologies with which one can determine the depth flow. These depth measurement technologies include mechanical floats, air bubblers, pressure sensors, ultrasonic sensors, capacitance sensors or radio frequency based devices.
However the average velocity is difficult to measure because the flow velocity through the cross-section of flow varies greatly. At the edge of the pipe, the velocity is close to zero and velocity increases quickly to a maximum and then decreases as it approaches the surface of the water or the far edge of the pipe.
Velocity measurement systems must operate over a large range of conditions. The typical sewer pipe has a diameter of 12 or less inches (30.5 cm) and normally has less than 2″ (5 cm) of flow. Depth of flow can vary from empty to full at velocities between slightly negative (<−2 ft/sec=−61 cm/sec) to very fast (10-15 ft/sec=3.05-4.57 m/sec). The largest pipes are normally not more than a meter or so in diameter with depth of flows varying from a several centimeters to a meter or so.
There are a number of systems that measure a point velocity in the flow and then predict the average velocity based on this measurement and the knowledge of the location of that measurement Montedoro-Whitney, Marsh U.S. Pat. No. 4,083,246 and Cushing U.S. Pat. No. 5,467,650. In sewers, such systems may have difficulty either if the sensor is placed too close to the edge (where all flows are zero) or may foul if placed to deep into the flow.
There are systems that make use of the Doppler Effect and the transmission and reception of ultrasonic energy into the flow to determine velocity. These systems fall into two classes, systems that utilize Continuous Wave (CW) or Pulsed ultrasonic transmissions. These classes further divided into systems that measure average velocity and systems that measure some intermediate indictor and then predict the average velocity based on that indicator. Such systems may utilize either Continuous Wave (CW) or Pulsed ultrasonic transmissions.
One example of the first class of systems, Nabity et al, U.S. Pat. No. 5,371,686, uses the transmission and reception of CW ultrasonic signals, Fourier transform processing on the received signals, and a process that uses the largest coefficient to normalize the signal and then averages certain weighted signals to produce a measure of the average velocity.
A distinct issue with such systems is that they interpret signal strength as a measure of velocity and then weight these numbers to provide an average velocity. Signal strength may be a measure of velocity but also correlates to size and reflectivity of particles. Other parasitic effects such as surface reflections, screening of distant particles by closer particles can also be a source of error.
Another system (Petroff U.S. Pat. No. 5,020,374 and Petroff U.S. Pat. No. 5,333,508) uses the transmission and reception of CW ultrasonic signals, Fourier transform processing on the received signals and various threshold and averaging techniques to determine the peak velocity in the flow. Average velocity is then determined to be approximately 90% of this peak value. One advantage of this approach is that it obviates issues associated with determining interpreting signal strength and mean velocity.
One significant limitation of such CW systems is that the received signal (generally very weak) must be sensed in the presence of the continuous and very strong transmitted signal. Not only does this make the measurement of very weak distant signals difficult, but it also makes it difficult to measure very slow flows. Very slow flows offer very little Doppler Shift and are therefore easily masked by the transmitted signal. Similarly, this makes it more difficult to distinguish between slow negative and slow positive flow. This problem is compounded if the sensor becomes fouled. In such cases, the coupling from transmit to receive can increase thus increasing useless crosstalk and while attenuating the signal received from distant particles. This is analogous to listening to music while holding a pillow over one's head and screaming. Useful information (music) is attenuated and the increased coupling by the pillow makes it more difficult to distinguish the scream from the music.
The second class of ultrasonic devices employs Pulsed Doppler transmissions and receptions. One such system (Petroff U.S. Pat. No. 5,226,328) describes a means of using pulsed Doppler to receive a signal from a selected volume of fluid some distance from a transducer and to then integrate that signal to produce a signal representative of the average velocity. Another example is an acoustic Doppler flow profiler described by Brumley et al. in U.S. Pat. No. 5,208,785 and subsequent fillings. Such current profilers measure the velocity in cells along a transmitted beam and then produce an average velocity estimate that is a function of the velocity in each of the measured cells. Such devices are known to work well in large pipes and deep flows but tend to be expensive and also have limited functionality if the flow is too shallow (i.e less than a few centimeters).
Therefore, there is a need for a flow sensor that accurately and economically measures flow velocity, including low flow and reverse flow, in a pipe over the full range of fill percentages without substantially interfering with the flow and may operate for extended periods in remote unattended locations.